architexts ISSN 1809-6298

183.05 climayear 16, aug. 2015

Climate change and it’s devastating impact on cities and urban life

The case of Mexico

Jan Bazant

abstracts

englishThe purpose of this research is to analyze the impact that climate change has on cities located in diverse latitudes and the devastating consequence on urban life due to their deteriorated environment.

In the last 60 years of Mexico, like any other Latin American country, has experienced the phenomenon of high population growth that quadrupled its population of 26 million in 1950 to 108 million in 2010; but also in the same period of time, the total urban population of the country increased substantially, from 26% in 1950 to 80% in 2010 (Fig.1). As a result, cities grew in size between 7 and 10 times compared to 1950, which meant a struggle by the growing population of all socio-economic levels for vital spaces within the cities - the most visible of which are the extensive low-income settlements. However, from 1990 the population of the country has begun to decrease and is projected to total 122 million by 2050, which calls attention to the fact that there will not be a significant population growth in the future, since 88% of the population already exists at present. This low population growth will reduce the pressure for a future urban expansion in most cities, but urban sprawl already exists in the cities, and the damage to their environments is already done.

Distribution of cities by size [Author based on CONAPO, La población]

To give an idea of the magnitude of the problem, Mexico is a country with a surface area of 1,959 million km2 and goes between the 14° and 32° latitude north. It has 10,000 km of coastline on the Pacific and Atlantic oceans, along with the Caribbean and the Gulf of Mexico, with a 12 nautical miles strip of patrimonial sea along its coasts (03). There are 4 cities with population of over 3 million inhabitants (the largest of which is Mexico City, with nearly 20 million inhabitants); 18 cities that have, or will have in the near future, 1 million inhabitants; 20 cities with population of more than 500,000 inhabitants; 84 cities with 100,000 to 500,000 inhabitants; 64 cities with 50 to 100 thousand inhabitants, 572 towns with 10 to 50 thousand inhabitants, and 711 villages with 5,000 to 10,000 inhabitants. The rest of the population lives scattered across the vast territory, in small villages of under 5 thousand inhabitants (02). It may be said that each geographical region has a hegemonic city around which the most important industrial and commercial activities are carried out, as well as the social equipment (hospitals, universities, markets), all of which complement the services of the nearby smaller cities, towns and villages; although each one has secondarily links with cities in other regions.

1.2 Bio-geographical and hydrological aspects of the territory

Mexico is a very mountainous country. There is a mountain range along the Pacific coast, and another one along the coast of the Gulf of Mexico. And in the central part there is a high plateau with an average altitude of 2000 meters above the sea level. It is important to emphasize this geological variation because it generates 2 major bio-geographical regions with their respective climates: the Nearctic ecozone, above the Tropic of Cancer (on the 27th parallel north), and the Neotropical below this latitude. The Nearctic zone, in which a dry, arid climate prevails, is located in the northern part of the country. On the other hand, the Neo-tropical regions are semi-arid and tropical, with a warm temperate climate (03). These considerations are important, because from these bio-geographic conditions we may infer which regions of the country are more vulnerable to drought and which to cyclones. However, the climate change has brought about fluctuations in those temperate regions of the central highlands that for centuries have had a stable climate, in terms of rainfall and weather, so that they are now equally subject to climatic disasters. Thus, today any region of the country is exposed to climate disaster of some sort.

Biogeographic regions of Mexico [Author based on INEGI, Regiones]

The country receives on average 1.515 hm³ of mean annual precipitation (or 1.515 million m³), of which 72% is lost to evapotranspiration, 22% drains superficially down rivers or riverbeds toward the sea almost unutilized, and only 6% is filtered to recharge the aquifers on which the urban population lives and which feed the country's economic activities - agriculture, industry and others. 04 shows that rainfall in the dry region of the north is less than 500 mm per year; in the temperate or semi-arid region of the central plateau precipitations range between 750 and 1500 mm annually, and in tropical regions they reach 3000 mm per year.

Levels of monthly average rainfall by regions [Author based on CONAGUA, Estadísticas]

1.3 Some physical-environmental characteristics of the cities

In order to understand better the configuration of the cities and their susceptibility to climate related disasters, we offer a scheme that seeks to summarize their most common geo-morphological characteristics. Most cities in the country were founded on crossroads of trade routes, on mountains rich in minerals and metals, near fertile lands for agriculture, and, of course, on seashore bays. They are generally located on lands with a slight slope, near a mountain range that provided them with water and had forests that were used for building and initially as fuel (firewood and charcoal). The relief of the terrain formed riverbeds for rainwater that descended from the mountains (05).

Typical characteristics of the shaping of cities [Author]

Thus, as the population and the cities grow, they do so in all directions, but initially they expand along the access roads. The expansion takes place both on agricultural land and on terrains with slopes and ravines, as well as on all types of soil -sandy, clayey, volcanic, and nitrous, among others, all of which eventually causes serious problems to the buildings and infrastructure networks in the cities. In the absence of an effective urban planning to curb the explosive urban sprawl, the cities expanded in a disorderly manner; but it has been observed that the light slope lands with the best locations were occupied by middle and higher income groups, while flat lands with soil problems and a difficult access were occupied by lower income groups (06).

Characteristics of urban risk areas [Author]

2. Concepts of threat, risks and urban vulnerability

2.1 Risks

The concept of risk refers to the probability of an urban center (people, buildings, infrastructure networks) to suffer damage or harm. In order for there to be a risk, there needs to be both a threat (or danger) and a population that is vulnerable to its impacts; "Vulnerability" being the propensity of a population, its buildings, its systems and networks, or the surrounding nature, to suffer damage. The risk is therefore a latent condition or potential, and its degree depends on the probable intensity of the threat and the existing levels of vulnerability. Consequently, threat and risk should not be considered as synonymous, for the degree of risk always depends on the magnitude of both the threat and the vulnerability (1). Threats may be identified as: natural hazards caused by the proximity of seas or lakes or socio-natural threats when events set “natural” limits to the development of cities (like floods).

2.2 Vulnerability

In contrast, while vulnerability entails a propensity to suffer damage, it is also a measure of the difficulties that a society must face in order to recover from the damage suffered. Vulnerability is socially constructed and is therefore expressed in terms of social insecurity. In the urban context, vulnerability is related to the structure, form and function of the city, and the characteristics and lifestyles of the various human groups that occupy the space. In order to reduce urban vulnerability, it becomes necessary to intervene in the conditions that generate it and repair the damage already done, which ends up being a never-ending process. The urban contexts where vulnerability is most apparent are: concentration, density and centralization, complexity and interconnectedness of the city and the low income settlements.

2.3 Types of urban risk

By risk we understand, then, the probability of exceeding a specific value of economic consequences, social or environmental in a particular place and time during a certain exposure. It is derived from relating the threat, or probability of the occurrence of a phenomenon, to a specific intensity, to the vulnerability of the exposed elements. The most common risks faced by the population and by a city are: biological risks, i.e., the proliferation of bacteria, physical-chemical risks such as air and water pollution, technological risks as industrial pollution and natural hazards like earthquakes.

2.4 Weighting of threats, vulnerability and risks based on study cases

Climate change has brought about a very severe impact on the lives of the population of Mexico, either because people live in arid climates with severe water shortages or because they live in tropical climates with too much water. And the Mexican high plateau, which used to have a comfortable temperate climate and sufficient water, is beginning to experience both the effects of a gradually increasing drought due to global warming and floodings from storms caused by the hurricanes that plague both the Pacific coast and the Gulf of Mexico. Table 1 summarizes the various threats, vulnerabilities and risks found in each one of these climates in their respective biogeographic regions, and shows that the country as a whole is under a growing threat from the climate change, with a high vulnerability due to the demographic concentration that has triggered an uncontrolled urban sprawl, thereby degrading the environment that surrounds them. Because they sprawl over all types of terrain, all the cities in the country have areas with a very high-risk of flooding, mudslides or subsidence affecting both the homes and the roads, as well as the functionality of their operating systems. And in cities located on areas with arid and semi-arid climates, the rise in temperature has been reducing the water reserves, which causes their entire populations to be at an increasingly high risk. The last column of the table indicates the examples selected to illustrate the impact that climate change has had on the cities in each biogeographic region.

To illustrate the above definitions and show the severe impact that the climate change is having on the population and their cities, some examples are provided. These case studies seek to be representative of the diversity and magnitude of weather hazards to which a city and its inhabitants are exposed. However, this does not mean that they cover the full range of threats and vulnerabilities.

4.1 Desertification processes

The desertification in the semi-arid zones that will gradually become deserts as a consequence of the global warming of the Earth is a large-scale process, with feedback and environmental interaction mechanisms that will have unpredictable consequences in the future.

These processes are measured by means of a soil study and show that the rise in global temperature is bringing a greater desertification of our territory, which leads to an increase in the erosive processes, the frequency and extent of forest fires, evapotranspiration, and salinity of the soil (2).

This decrease in the potential of the soil as support for biological functions leads to loss of biodiversity processes. In addition, as the organic layer of soil containing the nutrients dries out, the ability to recharge of aquifers is lost, and the conditions of human life in the cities located in these climates become increasingly critical. Consequently, the cities located in the arid zones of the northern border and the Baja California peninsula (with average temperatures of 35 to 40°C and more in the summer) and low levels of rainfall (less than 400mm per year) have a dry season lasting 8 to 12 months, while in cities located in the semi-arid northern high plateau (with average temperatures of 30 to 35 °C in the summer and 400-700mm of rainfall per year) (Figures 3 and 4) the dry season lasts 6 to 8 months. Their main vegetation types are xeric scrublands, acacia, cacti, grasslands, and halophytes. Both are permanently endangered and at a growing risk, for as the temperature increases, the environmental biodiversity deteriorates, and the water reserves of the cities diminish. Especially, recharge from the already scarce annual rainfall tends to decrease year after year, exerting further impact on the agriculture and livestock, which are beginning to perish.

Mexico is facing the worst drought in the last 50 years, 2011 having been the driest year since 1958, according to the National Water Commission (3), and the lack of rain has caused the drought to extend over more than half of the national territory. This drought is classified as strong or exceptional in 6 percent of the national territory, extreme in 15 percent, severe in 20 percent, and moderate in 13 percent, covering an overall 54 percent of the national territory. The states affected by the severest, so-called "exceptional" drought in the north of the country are the Baja California peninsula, Chihuahua, Coahuila, Sonora, and Nuevo León. San Luis Potosí, Guanajuato, Tamaulipas, the Federal District, the State of Mexico, Veracruz, Yucatán, Puebla and Tlaxcala have all suffered the effects of extreme drought (Figs. 7 and 8).

Drought in the countryside [Noticias MVS [MVS News], Nov. 18th, 2011]

To give an idea of the extent of this desertification process (03), this enormous arid zone in the north of the country suffering from "exceptional" drought has a population of 17,261 million people who live in: 4 cities with 1 million inhabitants each, 8 cities with 500 thousand to 1 million inhabitants, 10 cities with 100-500 thousand inhabitants, and 15 towns with 10 to 100 thousand inhabitants, as well as dozens of minor urban locations, within a vast region of around 639 thousand km2 (i.e. 32.6 % of the surface of the country). A true disaster for agriculture and livestock, which leaves them in a very highly vulnerable situation and risk of extinction from these economic activities. But in addition, the cities of the region face severe problems of shortage of water supply for consumption by their inhabitants and for maintaining their industrial and commercial production facilities (08).

For its part, thesemi-arid zone in the high plateau of the country (03), has a population of 20,775 millions living in: 4 cities with 1 million inhabitants, 10 cities with a population of 500 thousand to 1 million, 13 cities with 100 to 500 thousand inhabitants, and 15 cities between 10 and 100 thousand residents, plus dozens of minor localities within a territory of nearly 419 thousand km2 (i.e. 21.4 % of the country's territory). While the situation of the semi-arid zone is not as extreme as that of arid zones, it is nevertheless one of vulnerability and high risk, because, as a consequence of climate change, this vast region of the country tends to suffer from desertification due to the lack of rains (08).

Levels of drought in the country [Center for the Study of Public Finance, H. Chamber of Deputies. October, 2011]

As the country has increased in population, the availability of water has decreased. In 1950 the availability of water was 17.742 cubic meters per capita per year (m³/inhabitant/year), while in 2000 it dropped to 4.427 m³/inhabitant/year (4) (National Water Commission, CONAGUA 2008). While the population growth rate is declining, it is estimated that by 2030 the availability will drop even further, to 3.783 m³/inhabitant/year. Of course, this figure refers to the national average. Thus, the distribution of water within the Mexican territory imposes greater restrictions on the center and north of the country, which are the most urbanized and the fastest growing areas, located in the arid and semi-arid climatic regions where the availability of water is 1.734 m³/inhabitant/year, while the southeast, i.e. the region with the slowest economic growth, is constantly affected by tropical storms, so that the availability of water there is 13.097 m³/inhabitant/year (4).

4.1.1 Cities in arid areas: the case of Hermosillo, Sonora

Hermosillo is the most populous and fastest growing city in the state of Sonora; in 2000 it had 630 thousand inhabitants, in 2010 it had 749 thousand, and according to the most recent projections, it is expected to have nearly one million people by 2030 (5) (CONAPO 2006). However, its water resources are limited, and the city is a high-risk area because its aquifers are over-exploited and because it is located in an arid area where, due to climate change, the temperature tends to gradually increase, which leads to a worsening of the desertification process, with even less rainfall and further erosion and soil salinization.

In the 1970s, the city was supplied with water from the Abelardo Rodríguez dam, which had a capacity of 254 million cubic meters (Mm3) and was supplied by tributaries of the San Miguel and Zanjón rivers (Fig.9). However, due to the intense demographic growth in those years, it became necessary to resort to other sources; water began to be drawn from wells, and the city became dependent on a battery of wells located in several collection areas around the dam (the main were the Seri Plateau, the La Victoria communal land, and the Willard region, which was intended for the industrial zone and the Ford plant in Hermosillo). In this way, the city gradually became entirely dependent on underground sources for its water supply. What before was obtained by water flow through channels from the dam, is now drawn from aquifers at great expense of electrical energy (6).

[Center for the Study of Public Finance, H. Chamber of Deputies. October, 2011]

But the problem does not end there. Between 2000 and 2005 ground water also gave signs of diminishing. The water produced by the wells has been decreasing steadily; from June, 2004, to the same month the following year, the maximal expenditure for the wells that supply the city descended from 3.625 liters per second (lts/s) to 2.649 lts/s, i.e. , a 30 percent drop in just one year (7). In the face of these limitations and contractions of the sources of supply, the production of water for consumption by the city has greatly diminished. While in 1995 it amounted to a total of 95 Mm3per year, starting the following year it began to decline and was 87 Mm3in 1996, and 78 Mm3in 1997, minus a 38% of water loss due to leaks in the city pipelines. While there are no updated data on the reserves of these 100 wells, based on previous measurements, we may assume that these tend to continue to decline. And the question that comes up is: how quickly are they being depleted? If during the decade of the 1990s they dropped 17 Mm3 in two years, and considering that the 2010 – 2011 period has seen the worst drought in the country over the past 50 years (it simply did not rain in the arid or semiarid zones, CONAGUA 2011), one might assume conservatively that in the last decade the supply of these wells has dropped to 55 – 65 Mm3 per year. From this flow the 38% losses due to the leaks in pipes should be subtracted, so that the actual availability of water today in Hermosillo must fluctuate between 30 and 40 Mm3 per year. It is clear that as the temperature continues to increase due to global warming, this water reserve will continue to decline in the near future ––a critical situation for the region's inhabitants and agricultural activities.

At present, the Abelardo L. Rodríguez dam is virtually empty, as what remains at its bottom is but a small puddle of water with muddy sediments (09). This watershed is overexploited, and there is a competition for water resources between urban and rural producers up- and downstream (Moreno 2006). The water agency bought the rights over the Las Malvinas agricultural well upstream, in 2003, and downstream in 2005. An agreement was signed with the users of the Coast of Hermosillo, to transfer water from agricultural to urban use, whereby a new reservoir called The Bagotes was opened.

Despite the fragile water supply situation in the city, the resource has besides been handled inefficiently. The local operating agency, Agua de Hermosillo, turns over only 62 % of the fluid (53 Mm3), which implies that 38% (32 Mm3) is lost due to leaks and clandestine inlets. On the other hand, the fees do not cover the operation costs and much less allow the necessary investments to maintain the drinking water network. The current average rate is 5.30 pesos per cubic meter, and it has been estimated that to cover the production costs and make the supply sustainable, it should be at least 9.70 pesos (or $0.75US). To make matters worse, 28% of users do not pay, and there is no wastewater treatment plant in the city. This situation has persisted for several decades, and while the levels of unaccounted for water have been reduced (in previous years it had amounted to more than 50 percent), the growth of the city makes it necessary to increase the efficiency of the agency even further.

However, instead of searching for a more efficient management of the water, the tendency is to reduce the amount consumed through a rationing program that provides hours of service, called tandeos ["rounds"], first in 1998 and 1999, then in 2005, and more recently in 2010. In seeking new sources, the operating agency operator has raised the possibility of obtaining water from a desalination plant on the coast of Hermosillo, more than one hundred kilometers away from the city, and also from the neighboring basin of the Yaqui river, further away, the public works for which would entail high investment and operation costs (8).

But in addition to these administrative inefficiencies, the reality is that the water will be increasingly scarce for the inhabitants of the city. Here is why. Urban reserves in 2010 were 30 Mm3, which means that each inhabitant was supplied (if every home had an inlet and all the inhabitants consumed the same daily amount) about 110 liters per person per day (lpd).

But what will happen 20 years from now, by 2030, when the water reserves will continue to decline and, according to the projections of the CONAPO (2006), the population of Hermosillo will reach 1 million? By then the expense will probably be reduced to 24 Mm3, which divided among a population of 1 million inhabitants, would have an individual allowance of nearly 65 liters/person/day- lpd- representing a 40% decrease with respect to 2010. This allowance represents less than half of the standard minimum endowment of water per person (150 lpd).

4.1.2 Cities in semi-arid climate: the case of San Luis Potosí

The city is located on the central high plateau of the country bordering to the north with the vast desert areas. The city has an average annual temperature of 17.5 °C, while in summer is above 30 °C and in winter comes down to less than 10 °C with icy winds from the north. It has an average rainfall of 402 mm per year.

As most of the cities of the country, it has a high population growth, having increased from 650 thousand inhabitants in 1990 to 867 thousand in 2000 and to 947 thousand in 2010, and according to a projection by the National Population Commission (CONAPO) will reach 1.1 million by 2030 (CONAPO 2006). This rapid population growth has been the result of the considerable investment in industries and services that have generated employment and economic development but has fostered an uncontrolled urban expansion on the vast peripheries of the city.

In 1960 the urban sprawl covered 1, 760 hectares, and in 2000, 14 thousand hectares with a density of 62 inhabitants per hectare (inhab/ha); in 2010 it reached 20,000 ha, with a decrease in density, at 47 inhab/ha. This tendency to decrease in density means not only that the city is expanding horizontally, but also that between 35 and 40% of the urban sprawl of the peripheries is made up of vacant lots - i.e. speculative land that has some utility infrastructure and a growing added value. The intense urban expansion also contributed to the change in its growth pattern from a radially concentric model, based on the center of the city (prior to 1980), to a polynuclear type in which various unplanned "subcenters" of urban services arise, boosting the horizontal expansion of the urban sprawl further (9).

Location of the urban sprawl and aquifer No. 2411 (dotted line)
SEMARNAT et al (2005), Estudio [Drawing of the College of Saint Louis]

Raw sewage dumped into an open channel [Mileno Online, July 27th, 2010]

As in the rest of the cities of the country, the urban sprawl of San Luis Potosí has continued to expand up to the slopes of the San Miguelito mountain range, considered an aquifer recharge area. This is the result of a horizontal urban expansion process at very low density that has wasted the opportunity to populate the urban inlands more densely which already have utility infrastructure. This translates into inconsistencies in the urban dynamics that render it incompatible with the preservation of the aquifer and its natural environment; therefore, this expansion of the urban sprawl has itself become the main threat to the stability of the aquifer (010).

Besides, there are geological conditions that hinder the recharge of the aquifer. The San Luis Potosí basin comprises two aquifers: one shallow and one deep. The shallow or phreatic aquifer is made up of alluvial deposits, and its depth ranges between 5 and 40 meters. Due to its shallowness, it has a very dynamic behavior, and at times its chemical and bacteriological make-up shows high levels of pollution that render it unfit for human consumption. This pollution is the consequence of the dumping into open channels of both industrial waste and raw sewage from the city (011).

The deep aquifer is currently exploited by means of wells that reach depths of up to 350 meters in sedimentary material. The upper limit of the deep aquifer lies at a depth of approximately 100 to 150 meters in a granular layer; but at greater depth it is confined by a scarcely permeable sedimentary layer. Preliminary data about the age of water suggest that the deep aquifer is fed by very old waters of more than 1000 years, suggesting a low vertical recharge (CONAGUA 2005:48). That is to say, this sedimentary layer is made up of limestone clay that forms the bed of the aquifer, and its low permeability limits the recharge and flow through it. Hence, due to the demographic pressure of the city, more water is drawn from the aquifer than can be recharged.

There are no current data on the extraction of groundwater for urban uses in the Valley of San Luis Potosí; however, a water reservoir census was carried out in 1995-96 by the National Water Commission (CONAGUA or CNA). As for the 1995 geohydrological balance, the extraction volume was 110,273 Mm3/year, and the estimated recharge volume, 73.6Mm3/year, which results in a deficit of 36.66 Mm3/year. For the 2002 balance, published in the Official Journal of the Federation on January 31st, 2003, the available volume of groundwater extraction was shown to be 120.6 Mm3/year, and the total recharge volume, 78.1 Mm3/year, resulting in a deficit of 42.5 Mm3/year (SEMARNAT 2005, p. 31). According to the study by CNA (2000, p. 34) no volume is available to grant new concessions in the No. 2411 'San Luis Potosí' aquifer, which supplies water for the city.

In this way, based on official figures, it can be concluded that an accumulated annual deficit of 42.5 Mm3from the water reserves of basin No. 2411 amounts to roughly 30% of volume removed. That is to say, every 3-4 years, the accumulated deficit equals the volume drawn in the course of a year, which clearly shows the tremendously high risk to which the inhabitants of the city are exposed.

But work is under way on an alternative solution - the construction of the "El Realito" dam, located about 120 km away from the city and scheduled for completion in 2013. Its annual capacity will be 50 Mm3, enough to meet the demand of the city's population by 2030.

Finally, we should note that the major industrial plants of the city are supplied by 30 wells operating independently of those that supply the urban population. Of these, 26 are located within the same industries, and although they are franchised, their exploitation is difficult to monitor. While the remaining 4 wells are controlled by intermunicipal and state agencies, and on average between 300 and 700 thousand cubic meters/year are drawn from each well (SEMARNAT, op.cit. p.16), they are destined to serve the smaller industries of the city.

4.2 Impact of tropical cyclones

By contrast to the previous case, Mexico is one of the countries of the world most affected by tropical cyclones and is perhaps the only region that can receive effects of cyclones from two totally independent cyclogenic areas, the North Atlantic (coast of Veracruz, Tabasco, Campeche, Yucatán) and the Northeast Pacific (coasts of Chiapas and Oaxaca). This humid tropical zone covers around 253.812 km2(13% of the surface of the country) and with a population of 16,545 millions living in: 6 cities with 500 thousand to 1 million inhabitants, 8 cities with a population of 100 to 500 thousand, and 17 cities with 10 to 100 thousand inhabitants, plus dozens of minor urban localities ––a lower concentration of urban population than in the previous climatic regions.

In a tropical storm, winds are under 113 km/h (63 knots) and the pressure pattern includes at least two closed isobars. But tropical cyclones or hurricanes have winds of 114 km/h or more. In both the Atlantic and the Pacific Oceans, cyclones typically arise in the tropical sea from mid-May and June until November, although there have been years in which, uncommonly, they have appeared in December. Hence, hurricanes are formed mainly in areas of warm tropical waters, where intensity variations in the vertical wind shear are weak.

The tropical zone is characterized by having a hot humid climate with very short dry season, an average annual temperature above 22 °C that can reach 30 °C, an annual precipitation above 2.000 mm, and a relative humidity that fluctuates between 60 and 80 % but can reach 100% in the rainy season. Hurricanes appear after the summer rainy season and when the rivers and lakes are brimming, and the soil is soaked with moisture, to such degree that it can no longer absorb the rain. This is largely due to the fact that the vegetation cover of medium to high forests and savannas retains high environmental humidity levels.

The tropical zone located on the shores of the Gulf of Mexico and the south Pacific (see 012) produces an upward movement of air due to heating of the environment. This upward movement causes convectional rains. In addition, the tropical zone is the place of convergence of the trade winds coming from the sea and those that come from the sierra, which are often loaded with moisture and converge in one against the other. The meeting of the two masses of air produces an upward movement, and as they cool they cause the rains that fall throughout the year on this tropical coastal region.

But a cyclone, hurricane or tropical depression originates offshore by a closed circulation of air around a low pressure center that produces strong winds that run counter clockwise and abundant rain. Tropical cyclones draw their energy from the condensation of humid air, by the heat mechanism that feeds them and turns them into "core warm" storm systems. This warm core rotates like a whirlpool with great intensity of wind and rain storm and moves in the sea, sometimes heading toward a coast, where it penetrates causing great human and material damage along its path. Eventually, having no condensation of humid air to retro-feed it, it gradually begins to fade and lose strength, and the damage that it causes as it travels by land become therefore less and less severe, but covers a large territorial extension, affecting it with seasonal floods that last 2-3 days in coastal cities and the interior of the country. This way when a cyclone comes to ground, it has a devastating impact on the cities and population, caused by tidal waves of several meters in height which penetrate the coastal towns, often intense winds over 100 kilometers per hour (kph), with heavy rains that last up to a week.

The impact of hurricanes covers a vast territory, so there are three examples of the impact of a hurricane when it penetrates on a coastline (e.g. in the city of Villahermosa) and when it penetrates inland, with heavy rain affecting cities (cities of Morelia and Monterrey).

4.2.1 Floodings in coastal urban areas: The city of Villahermosa, Tabasco

The areas that are susceptible to flooding in the coastal strips according to the morphological characteristics of the relief are basically the coastal plains, swampy plains and basins with depressions. And these features are found on the entire coast of the Gulf of Mexico and the southern part of the Pacific coast. They are the extensive areas that are located between the mountain ranges and the coast, and consequently they are crossed by all the rivers that begin their course in the high mountain range and flow downward into the ocean. Therefore these plains contain rich alluvial lands, which makes them very fertile and suitable for agriculture and livestock.

Villahermosa´s Centla Marshes: rivers, mangroves and the coast of the Gulf of Mexico [Image captured from Google Earth]

Tabasco has a surface area of 24 thousand 661 square kilometers, 1.2 percent of the national total; and a population of just over 2 million, representing 1.9 percent of the overall population of the country. The flooding of Tabasco in 2007 is considered as one of the most serious natural disasters of Mexico in the last 5 decades, because it is located on a vast plain that is crossed by two of the mightiest rivers of Mexico, the Usumacinta River and the Grijalva. Both rivers come together in one only before its mouth. For this reason it turns into a huge swamp or marsh, known as the Centla Marshes , of enormous biological diversity (Figs.13)

In normal situations the rainfall in the basin of the Grijalva River ranges between 150 and 250 mm, but the heavy rains caused by a cold front and the presence of the Noel tropical storm in 2007 increased the amount of water that was falling into the basin of the Grijalva River including the North Chiapas -where are dams as important as the Angostura, Chicoasen, Malpaso and Peñitas, which generate the bulk of our electricity in Mexico. When the top of its storage capacity had been reached, Peñitas had to open its lockgates, increasing the flow of the rivers at a rate of 1500 to 2000 cubic meters per second, and its level of runoff increased to more than 1 meter above the normal. This finally enabled its overflow side toward the vast plains (Wikipedia, Inundación Tabasco [Tabasco flooding] 2007) (014)

The consequences were a flood of Villahermosa with its 715 thousand inhabitants (2010) - capital and main city of the state, and 80% of the state composed of rural areas and villages spread. 670 small urban communities were flooded, and 400 thousand people were affected. Although there was no loss of life, material losses were heavy, as affected families lost part of their household furniture, farmers lost their crops and their cattle was affected, and the general population was left practically without services (water, gas, electricity) during the month that this tragedy lasted. As the streets and roads were (totally or partially) flooded, there was no transportation, there were no schools, and the people could not attend to work on a regular basis, shop at the market or go to drugstores to purchase the required medications. The low-income population, who obtain their earnings through informal activities, saw their income dwindle severely. And as for sanitary sewerage, the sewers were flooded, and raw sewage became mixed with rainwater and spread across the territory.

Flooding of the city of Villahermosa, Tabasco [Wikipedia, Inundación de Tabasco, Flooding in Tabasco]

Villahermosa was the most affected, because more than half of the urban area was hit by floods, and the rest, which suffered minor damage, remained isolated by the flooding of the streets that give them access. The most affected neighborhoods were Las Gaviotas and La Manga, where the water level reached a height of up to 4 m, causing destruction of homes, cars, infrastructure and social equipment (schools, clinics,…).

But the damage is not only located within the city and in state of Tabasco but affected the whole coastal region of the Yucatan peninsula. Approximately 3.5 million inhabitants of the states of Quintana Roo, Yucatán and Campeche were left without LP gas, food and other commodities, since the access roads across Tabasco were flooded.

4.2.2 Floodings in low-lying areas: the city of Morelia, Michoacán

The other frequent case of urban disaster is one that happens in cities located in the central plateau of the highlands of Mexico, located at an altitude between 1500 and 2600 meters above mean sea level. Most of the cities in this region are surrounded by high mountain ranges that shape river beds that dislodge the seasonal rainwater. When the water level is low, these river beds remain dry, but when the "tail" of a cyclone comes to cross the mountain range, there is a period of heavy rains that lasts several days, concentrating the water in streams that overflow these dry river beds. Such is the case of the Morelia, in the state of Michoacán, whose north and southeast districts, into which the Chiquito and Grande rivers flow down from the mountains, become flooded every time there are exceptionally heavy rains.

Urban Area of Morelia, Michoacán [Image captured from Google Earth]

In recent decades, Morelia, like many other Mexican cities, has had an intense population growth, increasing its population from 430 thousand in 1990 to 780 thousand in 2010, projected to reach 1,100 millions by 2030. 62% of the nearly 10.500 hectares currently covered by the urban sprawl (Fig.15) consists of low-income settlements in the outskirts (27 %) and in the intermediate ring (35 %). Settlements of the intermediate ring have existed for more than 15 years, whereas those on the periphery are more recent.

These low-income groups occupy the cheaper lands, which are poorly suited for urbanization, i.e. which easily become flooded and have geological faults, ravines or unstable soils. While these areas have been identified for risk to urbanization, and according to the existing urban master plans are zoned for ecological conservation or right-of-way for federal water channels, the local government has been unable to contain the social pressure to occupy these lands.

The basin area of the city of Morelia is 1.200 km2, and has an average rainfall ranging from 398 mm to a maximum of 1208 mm per year. The most intense rainfall, ranges between 700 and 900 mm, occurs from June to September, and is caused by the invasion of masses of warm, humid air coming from the Pacific Ocean coasts of Michoacán; whereas the winter months are dry, with some winter rains due to some extraordinary phenomenon or to cold fronts coming from the north of the country. In the past 10 years has increased the average value of rainfall has increased and is now beginning to exceed 900 mm per year (10).

Under these conditions of high-risk land occupation, Morelia has been subject to floodings along these rivers during the last decade; as occurred in 2002 as a result of storms reaching 900 mm. Also the storms of 2003 and the dredging of the Cointzio dam (up basin) resulted in extensive flooding with more than 1000 mm of rainfall; in the year 2005 the rainfall came to exceed 1000 mm and caused damage to houses and infrastructure with its consequent impact of social equipment, so that it became necessary to evacuate the settlers of various neighborhoods and relocate them to shelters in the outskirts of the city (11).

The urbanization process has been extended to farmlands and old haciendas bordering the city, which has favored land speculation in the urban periphery. As a result of this, various settlements arose on the natural banks of the main rivers (Río Grande and Río Chiquito) and natural depressions where rainwater concentrates (Fig.16).

Based on the foregoing, the Rio Grande, with a flow rate of up to 70-90 m³/sec during the rainy season, and the Rio Chiquito, with up to 70-80 m³/sec during tropical storms, are considered to be in high risk due to constant overflows. Additionally, in recent years, floods in the outskirts of the city have caused countless physical damages and economic losses for their low-income residents, whereby it is clear that the unceasing urban expansion and the proliferation of irregular low-income settlements have favored the emergence of new high-risk areas.

Mud-filled streets after a flood in a consolidated working-class neighborhood of Morelia

Typically the consequences of floods due to the overflowing of rivers (in dense, consolidated urban areas) and of floods in the new peripheral settlements (on land depressions in low-density urban areas with widely scattered houses) in extreme cases affect more than 60 thousand inhabitants or around 11,000 dwellings, as in some parts the floods reach depths of more than 70 centimeters. 57 neighborhoods with very high, high and medium risk have been recorded (La Extra, p. 1), although only 41% of these are in a very high or high risk of periodical flooding and amount to nearly 9,000 homes, in which 35 thousand inhabitants dwell, as shown in 018. In addition, located within the floodable perimeter are 531 urban facilities that provide various services, including social assistance, power supply, business, communications, cultural, sports, educational, recreational, financial and government services, to the inhabitants of these neighborhoods and to the rest of the city. These facilities are rendered inaccessible to the population by the floods, and some of them also become flooded. Notably, 146 are educational institutions that exist in the city, of which 27 are located in areas with high risk of flooding, endangering little more than 2 thousand 200 students.

It is obvious that a rainstorm not only causes the flooding of streets and houses in areas with high and very high risk and material damage to the houses and household property; but the true disaster for the inhabitants begins when the water level descends, leaving in their homes a layer of contaminated sludge that needs to be removed. And the same way, the streets are left with a layer of up to 15 cm of sludge that will need to be removed with mechanical equipment (017). Once this is done, the entire sanitary sewer system needs to be dredged, having been rendered useless by the seepage of sludge from turbulent storm water run-offs. And it is not until the sanitary lines are reasonably clean (after several weeks), that the affected inhabitants can return to use their bathrooms and kitchens as usual. A real tragedy!

Floods in Morelia are a complex problem that has been increasing as the city grows. The occupation of strips of land adjacent to the river beds, the accumulation of solid waste discharged on the dry river beds (when there is no rain), the construction of houses next to riverbeds, which weakens the banks and makes it impossible to give them maintenance, the insufficient capacity of the sewerage network to drain the maximum rain water levels, historic deforestation over the whole basin and the uncontrolled urbanization on the whole periphery, whether or not it is suitable for occupation, favor a rapid concentration of water in the low-level areas that has increased the vulnerability of Morelia to the flooding caused by the extreme rains.

4.2.3 Urban areas invading 'dry' riverbeds: the case of Monterrey

The metropolitan area of Monterrey is surrounded by the Sierra Madre Oriental and shows almost vertical cuts in the mountains in the southern zone and part of the east and west (see 018), which produce high slopes on very little permeable rocky soils whose geological features which make them prone to capture and concentrate rain water in normally dry riverbeds, directing it toward the lowest parts of the city and thereby causing flooding.

As the metropolitan area of Monterrey has increased its population to almost 4 million inhabitants (2010), a large part of the high- and middle-income sectors have been settling on the hilly sides of the sierra, which afford better weather and great views over the city and the mountain range, compared to the northern plateau, where the industries and working-class neighborhoods are located. And in the absence of a more strict implementation of the Urban Development Plan of the Municipality, a large part of the "dry" riverbeds and the surrounding slopes of the sierra have gradually been urbanized. However, the geological substrate of these rocky massifs shows weathering, erosion, surface fractures and roughness with a large amount of cracks. Because it is a mountain range with a 600 m high, nearly vertical rocky wall, multiple small riverbeds remain "dry" during much of the year.

But when a cyclone penetrates inland, its winds, coupled with torrential rain, crash against this huge rocky wall and begin to accumulate torrents of water during the 2-3 days that the storm lasts. The first day, the rain water saturates the cracks and the little topsoil on the slopes, and from the second day on it begins to flow increscendo down the slope. Thus, the storm water finds the natural slopes with their ancient riverbeds, which are now urbanized, and overflow onto the surrounding areas and destroy everything along their way (018)

One of the most recently occurring events was the hurricane Gilberto (category 5), in September, 1988, which generated 3 days of intense rainfall, causing flood waters to run off at a rate of almost 4.400 cubic meters per second (m³/s) along these dry riverbeds and gather in the usually dry bed of the Santa Catarina river, which is nearly 11 meters deep and has an average width of 100 m, dragging along several buses that were crossing the bridges over it (12). The problem was partially solved through the building of the "Rompepicos" dam, upstream of the city of Monterrey. As its name indicates, its role is to dispel the storm water run-offs generated in the upper basin, and to reduce the volume of water and dragged materials, which must be held in check and subdued in the basin prior to their arrival in the city of Monterrey. This reduces the intensity of the flood water run-offs, but not their volume, which in the end also wreaks havoc in the city of Monterrey.

A visualization of the water torrents descending upon the developments.

In the month of July 2010, the "Alex" tropical storm (category 2) struck the region, causing serious erosions of the main rivers of the region, such as Santa Catarina and Pablillo (Linares). In addition, the whole drainage network that makes up the hydrological basins suffered extensive erosion due to precipitations of more than 800 mm accumulated in 72 hours (13). The overload of water from the Sierra Madre Oriental as a result of the "Alex" storm brought about considerable mass movements on the main roads of the state of Nuevo León, as well as the collapse of karstified areas of the same mountain range.

Figure 20 show the disastrous consequences of having allowed the urbanization of the "dry" riverbeds of the mountain as streets in developments, because when an event concentrating high volumes of water in this subbasin occurs, then flood water run off downhill with great force along the original riverbeds, destroying streets and houses in their path. Not to mention the hundreds of cars. Fortunately, on this occasion there was no loss of human lives.

A visualization of what happened is shown in 019. The estimated surface area of this subbasin is 665.000 m2, which are multiplied by 800 mm and divided by 3 days to obtain an approximate volume of water per day (1.773 m³). Judging by the destruction of the streets, this flow was divided between the 6 natural riverbeds in this section of the mountain range known as Mitras. As a result, each of these "dry riverbeds" concentrates a particular downhill flow, according to its specific conditions (such as catchment surface, width, length, and slope, among others). Initially in the first day the runoff was gentle, but on the second day, once the soil had become saturated with water, it became stronger and began to wreak havoc. By the third day of rain, the flood waters continued to run along each riverbed, increasing the damage and dragging large volumes of rubble downhill, as shown in 020. Finally as the cyclone loses its strength and the rain ceases, the runoffs gradually subside until they disappear. Although, the material damages consisted basically of the destruction of the streets, it took weeks before the residents could gradually enter and exit their homes, and more than a year to remove the tons of debris in lower parts of the streets and to re-enable the streets for vehicular traffic.

While these natural disasters have befallen on Monterrey in periods of 20 years or more, with the climate change, these cycles will escalate in a quite unpredictable manner. And very likely, the cycles of repetition will become shorter. Twenty years ago all these developments on the sierra did not exist, and therefore these dry riverbeds served their purpose to drain the rain water downhill. But today, with all of the low part of the sierra already urbanized and inhabited, having experienced in 2010 the harrowing effects of a category 2 hurricane and still being liable to suffer a future impact of another category 5 hurricane like 1988's Gilbert, this area should urgently be declared very high risk by the local government, and harsh disaster prevention regulations should be stipulated for the security of their inhabitants.

5. Concluding remarks

Urban disasters derived from climatic change phenomenon, are extensively reported by news media during the days the tragedy lasts. As we know, they dramatize the tragedies with pictures with scarce content, so when the climatic phenomenon is gone, no real knowledge can be derived from these damaging experiences, which could help mitigate future events.

This article analyzes different urban tragedies related to climatic change in order to present a more articulated view of the severe consequences they could happen again in the same cities or in any of the rest of the nation´s cities, in order to become more aware that this climatic change is very slowly increasing in intensity and in their consequences.

So, on one side we have this climatic change phenomenon which has been largely analyzed, but in the other we have all this intense urban growth around cities whose consequences we are starting to analyze nationwide.

While these bio geographic regions have always existed in Mexico, only 35 years ago the country's population was half what it is today, and the urban sprawl of the cities was 60% less than today, and therefore the risks were also lower. Therefore, natural events were less frequent and less intense because the climate change was only just beginning. In 1980, the urban sprawl had not yet massively invaded those high-risk areas as gullies, hillsides, low-level areas or aquifer replenishment areas, as in subsequent decades up to the present day. There is no doubt that this uncontrolled urban sprawl in all the cities across the country has substantially degraded their natural environment, have made them more vulnerable and have intensified urban risks through the occurrence of these climatic events.

But cities have interconnectivity of urban systems - such as transportation or infrastructure- so when a natural disaster happens in any area around it, it has an impact on the rest of the city, such as big traffic jams for the flooding of avenues or turnpikes, power outages, lack of water supply, delay in the collection of garbage, obstruction of sewers, among others. Thus, a natural disaster comes to affect virtually the entire population of a city directly or indirectly with equal or lesser intensity.

As we know, those most affected within a natural event are the lower income groups that represent around 50% of the population of cities; and which, due to their low purchasing power, settled on cheaper land presenting the highest environmental risk. The irony is that when a natural event of any sort occurs, it is these low-income sectors that in the end must pay for the reconstruction of their houses and their replacement of their scant material goods and household items. And this enormous effort to bring their lives and their habitat back to normal makes them liable to suffer again a similar disaster in the near future; in spite that the government provides help in case of emergencies and eventually reconstructs the urban damage occurred.

Apart from the physical devastation on the population and the structure of the city; the population is vulnerable to health and biological risks (epidemics) that may arise, but this is an uncertainty that can only occur if there is an oversight on the part of the authorities and they don't rescue the human lives in danger, provide care for the injured, and remove the dead, and especially if they fail to collect the organic garbage likely to rot.

In spite the important role in rescuing and saving human lives before big cyclones and earthquakes occur by relocating population in safe places and offering them temporary shelters, the federal government agency National Center for Disaster Prevention (Centro Nacional de Prevención de Desastres or CENAPRED) can do little with the everyday hazards that occur during rainy seasons or the disastrous consequences of unexpected change of rainfall intensity or duration within the city or its region. And since cities have expanded irrationally along their perimeter, it is practically impossible to prevent climatic related disasters within an urban area.

But the situation of the arid zones is even worse, because the gradual increase in temperature is making living conditions very miserable for all their inhabitants, regardless of their income level. The lack of rain fall and the evaporation of soil humidity which keeps vegetation alive is drying the aquifers and water is becoming more scarce every day. It is not a matter of how much it costs to pump it out, because not much is left. Aquifers are overexploited and water consumption is becoming rationed, so living conditions are deteriorating gradually.

The key implication is that in order to reduce the high risk of these urban disasters, large investments on prevention actions (of different kind) should be implemented in all cities, being evident that due to the magnitude and complexity of the climatic change nation-wide problem, this is not likely to happen in the near future, so the urban population will likely continue to suffer these damages. Curiously enough, federal government or international resources only become available when the disasters occur, not before.